import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int pocky = sc.nextInt();
		int longP = sc.nextInt();
		sc.close();
		if(pocky%(longP*2)==0){
			System.out.println(((pocky/(longP*2))-1)*longP);
		}else{
			System.out.println((pocky/(longP*2))*longP);
		}
	}// mainmethod

} // clas